1. Field of the Invention
This invention relates to a method for growing nitride III-V compound semiconductor layers and a method for fabricating a nitride III-V compound semiconductor substrate and, in particular, to those suitable for use in a semiconductor device, such as semiconductor laser, using nitride III-V compound semiconductors.
2. Description of the Related Art
Recently, there is a strong demand for development of semiconductor lasers for emitting green, blue or ultraviolet light with a short wavelength, derived from the demand for optical discs and magneto-optical discs permitting higher recording and reproducing densities and higher resolutions.
Nitride III-V compound semiconductors represented by GaN, AlGaN and GaInN are known as appropriate materials for fabricating semiconductor devices capable of emitting short-wavelength light (for example, Jpn. J. Appl. Phys. 30(1991) L1998).
Growth of nitride III-V compound semiconductor layers conventionally relies on metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), and uses a sapphire (Al.sub.2 O.sub.3) substrate or a silicon carbide (SiC) substrate as the growth substrate.
However, nitride III-V compound semiconductor layers grown on a sapphire substrate or a silicon carbide substrate are liable to contain defects or to crack due to a difference from the substrate in lattice constant and thermal expansion coefficient. Additionally, when a semiconductor laser is fabricated by stacking nitride III-V compound semiconductor layers by growth on a sapphire substrate or a silicon carbide substrate, it is difficult to make its cavity end surface by cleavage.
These problems will be solved if a nitride III-V compound semiconductor substrate is obtained. That is, nitride III-V compound semiconductor layers grown on a nitride III-V compound semiconductor substrate eliminate the problems of defects or cracking because their lattice constant and thermal expansion coefficient coincide with those of the substrate. Also when a semiconductor laser, for example, is fabricated, its cavity end surface can be made by cleavage. In addition to these advantages, the use of a nitride III-V compound semiconductor substrate enables an electrode to be made on the bottom surface of the substrate, and therefore enables high-yield production of highly reliable semiconductor devices, e.g. semiconductor lasers.
However, since the vapor pressure of nitrogen is high, none of typical methods for preparing Si substrates or GaAs substrate can be used for fabricating nitride III-V compound semiconductor substrates.
In addition to MOCVD and MBE, hydride vapor phase epitaxy (VPE) is also known as a method for growing GaN. Hydride VPE can grow a GaN layer as thick as several micrometers to hundreds of micrometers per hour and is considered one of most effective methods for manufacturing GaN substrates. Regarding fabrication of GaN substrates, there is a report that GaN layers were grown on sapphire substrates, GaAs substrates, and so on, by hydride VPE. These GaN substrates, however, do not have satisfactory qualities for use as substrates, due to unacceptable crystallographic properties or surface conditions, or their GaN layers being grown diagonally, and not normally, with respect to the substrates.